This function should do the trick if you want to specify a particular sheet. I took the solution from user6432984 and modified it to not throw any errors. I am using Excel 2016 so it may not work for older versions:
Function findLastRow(ByVal inputSheet As Worksheet) As Integer
findLastRow = inputSheet.cellS(inputSheet.Rows.Count, 1).End(xlUp).Row
End Function
This is the code to run if you are already working in the sheet you want to find the last row of:
Dim lastRow as Integer
lastRow = cellS(Rows.Count, 1).End(xlUp).Row